1. Field of the Invention
The invention relates to the field of wireless communications, in particular to transmit power control and link adaptation techniques and mechanisms.
2. Background Information
The IEEE 802.11 is a wireless LAN (Local Area Network) standard that has been standardized by IEEE (Institute of Electrical & Electronics Engineers). The IEEE 802.11 wireless LAN standard is currently undergoing a process of extending the standard with QoS (Quality of Service) features. The objective is to enable, for example, computers or multimedia devices to communicate under QoS constraints. This standard extension goes under the name IEEE 802.11e and is managed by the so-called task group e, TGe.
Recently, the IEEE 802.11 standard was also extended with a new physical layer allowing higher data rates than the previous physical layer. Various data rates are enabled through several code rates and signal constellations. The purpose is to allow link adaptation depending on the channel quality. The high rate PHY (physical layer) on the so-called 5 GHz band is called IEEE 802.11a and is based on OFDM (Orthogonal Frequency Division Multiplexing). The corresponding so-called 2.4 GHz band PHY is called IEEE 802.11b and uses single carrier modulation schemes.
IEEE 802.11 operates either in a DCF (Distributed Coordination Function) or a PCF (Point Coordination Function) mode. The former is for distributed operation and the latter for centralized control from an access point, AP. So far the PCF mode has not been ratified by implementers as the complexity is consider to high, instead DCF is used both for the distributed operation as well as with the AP.
The origin of IEEE 802.11 access scheme is traced back to BTMA (Busy Tone Multiple Access) which was the first proposed method for distributed control of channel access avoiding the well known hidden terminal problem.
In MACA (Multiple Access with Collision Avoidance), proposed by Phil Karn in 1980, the introduction of a Request To Send (RTS) and Clear To Send (CTS), handshake phase prior data transmission solved the idea of distributed reservation. This presented a more feasible basis to build a practical system upon as it did not divide the frequency band in a channel for data and busy tones, as in the BTMA scheme. Also the idea of random exponential back off, that was later used in IEEE 802.11, was introduced in MACA.
In MACAW (Multiple Access with Collision Avoidance for Wireless), the basic mechanism of MACA was refined. Among other things, a link acknowledgment, ACK, scheme was introduced. The access scheme of IEEE 802.11 is now based to a great extent on principles developed in MACAW.
Other ongoing standardization activities in IEEE 802.11 include the so-called TGh (Task Group h, i.e., an IEEE task group for IEEE 802.11h) that has the objective of designing and including transmit power control (TPC), as well as distributed frequency selection (DFS), in IEEE 802.11a. The purpose of power control from a standardization point of view is primarily to enable IEEE 802.11a stations, STAs, to conform to European regulatory requirements.
As background information, the basic access principles for IEEE 802.11 will now be described. For more detailed information the reader is referred to the standard IEEE 802.11-1999 (which replaces IEEE 802.11-1997), the standard IEEE 802.11a-1999 (High data rate on the 5 GHz Band), and the standard IEEE 802.11b-1999 (High data rate on the 2.4 GHz Band). Good and simple overviews may also be found in a) “Smart Antenna Systems and Wireless LANs”, authored by Garret T. Okamoto and published by Kluwer academic publishers (ISBN 0-7923-8335-4), and “IEEE 802.11 Handbook, A Designers Companion”, authored by Bob O'Hara and Al Patrick (ISBN 0-7381-1855-9).
There are two modes of channel access scheme operation in the Distributed Co-ordination Function (DCF), one based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) and one based on CSMA/CA including RTS-CTS message exchange. A MIB (Management Information Base) attribute “dot11RTSThreshold” is used to differentiate the use of the two. MPDUs (MAC Protocol Data Units, where “MAC” stands for Medium Access Control) shorter than the threshold is sent without RTS-CTSs, whereas longer MPDUs are sent with RTS-CTSs. The focus here is the RTS-CTS based CSMA/CA mechanism that enables mitigation of hidden stations and hence in general provides a more efficient use of the wireless medium.
FIGS. 1A-1D show a communication procedure between a station T and a station R, and related effects on nearby stations E, F, G, H. In FIG. 1A, station T transmits an RTS (Request to Send) signal to the station R. The transmit range 102 of the station T encompasses the stations R, E and F, but not the stations H, G. Thus the stations R, E and F receive or overhear the RTS signal, but the stations H, G do not. In a next step shown in FIG. 1B, in reply to the RTS signal, the station R sends a CTS (Cleared to Send) reply signal to the station T. As shown in FIG. 1B, the transmit range 104 of the station R encompasses the station F, H but not the stations E, G. After receiving the CTS signal, in FIG. 1C the station T transmits a DATA signal to the station R, and then in FIG. 1D the station R acknowledges receipt of the DATA signal by sending an ACK signal or message to the station T.
Since the station H is a hidden station with respect to the station T, it is informed of the intention of station T to transmit via the reply CTS message sent by the station R (since station H is not hidden from the station R, i.e., it is within the transmit range 104 of the station R). As a consequence, the station H will not transmit and disturb ongoing reception by the station R. Stations E and F will in a similar manner defer channel access to the stations T and R, after overhearing the RTS from the station T and/or the CTS from the station R. As shown in FIGS. 1A-1D, station G is hidden from both stations T and R, and therefore will likely not overhear the RTS or CTS, and therefore it may transmit.
FIG. 2 illustrates frame formats used in IEEE 802.11, where the numbers above the boxes indicate the size of the information in the box. Note, Address 4 in the DATA and MANAGEMENT frame exists only for DATA frames in a wireless DS (Distribution System), and does not exist in MANAGEMENT frames.
FIG. 3 illustrates the frame exchange including RTS and CTS. When frames are received by stations other than those intended to receive the frames, a so called NAV (Network Allocation Vector) is set according to a duration value indicated in a field of the frame. This provides an additional collision avoidance mechanism to the physical channel access sensing and is therefore called virtual channel sensing. As long as either the physical or virtual channel sense indicates activities on the channel, a station must remain silent. When the channel becomes free, stations start contending for the channel according to the channel access principles defined in the IEEE 802.11-1999 standard. In general, the NAV can only be extended if new frames are received. There exist some special instances when the NAV can be reset as well, but that is not the normal operation.
FIG. 4 illustrates use of RTS-CTS with DATA fragmentation. Each fragment and ACK then acts as implicit RTS and CTS. Additional fragments are indicated by a bit (field) in the frame control of the fragments.
According to the IEEE 802.11-1999 standard, CTS should be sent with the same link rate as RTS, and ACK should be sent with the same link rate as DATA. The original purpose is to enable the originating or transmitting station (e.g., the station T of FIG. 1) to calculate the duration value prior to RTS transmission.
FIG. 5 shows a detailed example of two stations attempting to access a channel through the RTS-CTS phase. In FIG. 5, each time slot=9 microseconds, the SIFS (Short Inter-Frame Spaces) time=16 microseconds, a CCA (carrier sense) time<4 microseconds, a min CW (Contention Window)=15 time slots, a max CW=1023 time slots, an air propagation time<<1 microsecond (in FIG. 5, it is 0 microseconds), DIFS=SIFS+2 time slots=34 microseconds, RTS=52 microseconds @ 6 megabytes/second (RTS=24 microseconds @ 54 megabytes/second), and CTS=44 microseconds @ 6 megabytes/second (CTS=24 microseconds @ 54 megabytes/second).
International Publication No. WO-9501020 A discloses that each station in a wireless LAN (Local Area Network), using time-distributed multiple access control, listens to traffic using the network communications channel, for example, for spread-spectrum, frequency-hopping transmissions. Each station constructs its own network allocation vector from the received transmission contents, indicating when the channel will be in use. Message transmission uses four-way handshaking with two short control packets, “Request to send” (RTS) and “Clear to send” (CTS). The RTS packet includes the data transmission length, enabling the various receiving stations in the network to reserve and block their use of the communications channel over the period of time concerned. The CTS packet repeats this data length, for the benefit of receiving stations not within range of the source transmission. This document corresponds to the IEEE 802.11 standard defined in the IEEE 802.11-1999 standard.
Some ideas regarding transmission power control in DBTMA (Dual Busy Tone Multiple Access), are described in S.-L. Wu, Y.-C. Tseng, and J.-P. Sheu, “Intelligent Medium Access for Mobile Ad Hoc Networks with Busy Tones and Power Control”. Int'l Conf. on Computer Communications and Networks, 1999, pp. 71-76. DBTMA is an extension of BTMA with dual busy tones instead of a single busy tone.
However, power control is not supported in known RTS-CTS based channel access schemes.
With respect to DBTMA with TPC, BTMA (Busy Tone Multiple Access) as such is generally not a viable solution for distributed channel access as it is extremely unpractical. It is merely used as a simple system to study in the academic literature. Also, control messages use maximum Transmit Power (TP), and therefore it is not possible for control messages to share a channel with data traffic as that would cause harmful interference peaks for data reception. Another drawback is that information regarding fixed TP is assumed known at the receiver. In addition, DBTMA with TPC only attempts to solve a problem in a specific situation, namely in a distributed system where stations are neither associated with APs, nor associated in a group with other stations. Another drawback is that asymmetries in interference, link gain, or TP capabilities are not been considered.
There are also additional problems common to each of general RTS-CTS, IEEE 802.11 and DBTMA, namely a) link adaptation has not been considered in the RTS-CTS framework, and b) asymmetries in terms of link adaptation capabilities have not been considered.